During normal development, organ growth is tightly regulated such that a high degree of fidelity to both size and shape is maintained. Often this constancy is achieved by a robust arrest of growth once final size has been reached. In response to injury, however, some organs are able to engage in additional growth, thereby regenerating the lost or damaged tissue. Regenerative capacity varies widely across the animal kingdom. Some organisms can replace lost appendages, such as limbs or tails, restore damaged internal organs, such as the heart or liver, or even regenerate their entire body plan from a small fragment. Other animals, notably humans, exhibit markedly reduced regenerative capacity. Damaged tissue is replaced by a scar and organ function is accordingly lost or reduced. Why some organs and organisms possess impressive regenerative abilities, while others lack this seemingly beneficial trait remains largely unresolved. Improving our understanding of how regeneration occurs in the former may inform therapeutic efforts to improve regeneration and repair in damaged human tissues.
Tissue damage elicits local and systemic responses that determine the efficacy of the ensuing repair and regeneration. Locally, damaged or dying cells are removed and additional tissue growth or remodeling must be undertaken to restore tissue integrity. On the systemic level, injury can lead to the recruitment of immune cells and the activation of an inflammatory response. Long-range signaling from the wound site may also broadly impact animal physiology by affecting processes such as metabolism. Reciprocally, regeneration may be influenced by organismal factors, such as age and nutrient availability. Very little is known about the cellular and molecular mechanisms underpinning these various aspects of regeneration.
We have used the regenerative capacity of Drosophila larval imaginal discs to investigate the local proliferative and the systemic immune responses to tissue damage. Chapter 2 describes a system that we have developed to produce tissue damage and regenerative growth in imaginal discs. Using genetic mosaics we generate clones of cells that carry a temperature-sensitive cell-lethal mutation. We can thus selectively ablate these cells upon shifting to an elevated temperature and assess the regenerative capacity of the surviving sister clones. This system has allowed us to conduct a genetic screen for recessive mutations that selectively impair regenerative growth. Using traditional methods and whole-genome re-sequencing we have identified several of the mutations isolated in our screen. We have found that regenerative growth occurs, in part, by an acceleration of tissue growth and thus appears to sensitize cells to the loss of factors generally required for more rapid growth. In this way, regenerative growth may share features with some types of overgrowth. Indeed, two of our mutations are common targets for anti-cancer drugs.
In Chapter 3, we investigate the innate cellular immune response to tissue damage in Drosophila. Using ionizing radiation to generate widespread apoptosis in proliferating larval tissues, we find that broad changes in the blood cell population ensue. These include macrophage activation and the differentiation of lamellocytes, an otherwise cryptic cell fate. Using the power of Drosophila genetics, we consider a model in which the engulfment of apoptotic cells by activated macrophages allows these cells to stimulate a broader, inflammatory-like immune response.
